This invention relates generally to communication systems. The present invention relates more specifically to reducing peak to average power ratios in single carrier and multi-carrier communication systems.
In recent years multi-carrier communication systems have received more attention. Multi-carrier communication systems offer the promise of increased bandwidth combined with two-way communications.
However, several problems still remain to be solved to ensure the widespread use of multi-carrier communication systems. One concern is how to reduce the peak to average power ratio of a multi-carrier transmission.
Referring to FIG. 1, a multi-carrier transmission is composed of a number of independent signals. FIG. 1 is a frequency domain plot of several signals 10(1)-10(n). Each signal 10(1)-(n) is centered a different frequency f(1)-f(n). Often times the frequencies are equally spaced apart. The frequencies are commonly referred to as carrier frequencies.
In most multi-carrier communication systems the signals 10(1)-(n) are combined together as a vector. An inverse fast fourier transform (IFFT) is usually performed on the vector to produce a discrete time domain signal which is converted to a continuous time domain signal and transmitted. FIG. 2 illustrates a continuous time domain representation of a typical output signal 30 of a multi-carrier transmitter.
Signal 30 contains a number of peaks 31-34. A problem with the output signal is that the peaks 31-34 often times exceeds the output capabilities of the transmitter. If the transmitter is only capable of transmitting at amplitudes of up to +/-10 dB, the peaks saturate the transmitter and the peaks are cutoff in the transmitted signal. Saturation causes the transmitted signal to lose a significant amount of information, which may or may not be corrected for by the receiver. Thus, it is important to reduce the peaks in order to maintain the integrity of the transmitted signal.
Reducing the peak to average power ratio of a signal requires that the number and magnitude of the peaks are reduced. There have been several attempts to reduce peak to average power ratios, although they are only successful to a certain extent.
The placement of the different signals 10(1)-(n) at different carrier frequencies f(1)-f(n) affects the shape of the output signal 30. One method randomly shuffles the phase of the signals 10(1)-10(n) at each carrier frequency f(1)-f(n). Random shuffling does not completely eliminate the problem, although randomizing has been shown to somewhat reduce the peak to average power ratio to an extent. Random shuffling also requires performing an additional IFFT. In addition to not completely reducing the peak to average power ratio to a practical point, that particular method also requires that additional information, side information, be sent along with the transmitted signal. In order for the receiver to be able to decode the transmitted signal the receiver must also know how the signals 10(1)-10(n) were randomized. Thus, the randomization scheme requires extra bandwidth to transmit the side information and does not effectively reduce the peak to average power ratio.
Another method has been applied to multi-carrier communication systems that use a small number of carrier frequencies. In that method all the different possible outputs of each signal 10(1)-10(n) are simulated. For example, if each signal 10(1)-(n) is a 4-ary quadrature amplitude modulated signal, each signal would be one of four different waveforms. If there are ten carrier frequencies, then over a million combinations are simulated. Those combinations of the outputs of signals 10(1)-(n) that exhibit peak to power ratios that exceed a specified limit are not used in actual transmissions. Typically, a channel must be simulated periodically because of changes in the channel's characteristics.
The elimination of some of the possible combinations of the outputs of the signals, however, reduces the bandwidth of the communication scheme. Further, the method can only be applied to communication systems that use a few carriers since the number of simulations required increases exponentially with an increase in the number of carriers. That is, if M-ary QAM and N frequencies are used, N.sup.M combinations must be simulated. M can be as high as 1024 and N even larger. Thus, this method becomes impractical when even a moderate number of carriers are used.
A third method involves performing inverse fast fourier transforms on subsets of the signals 10(1)-(n). For example, an IFFT may be performed on the first one fourth signals, another IFFT for the second one fourth, and etc. The four output signals may then be linearly combined to provide one output signal. Reducing the number of carriers within a single IFFT output reduces the peak to average power ratio for that output signal since there are fewer signal components. The linear combinations are compared to determine which combination has the best PAR.
As the number of signals and carriers increase the number of IFFTs that must be performed on the subsets of the signals increase, according to the number of signals incorporated within a single IFFT. The complexity of the transmitter thereby increases by the number of IFFTs that must be performed, compared to a single IFFT. Further, information about the linear combination of the transmitted signal must also be passed along to the receiver. This information is even more vital, and usually requires additional bandwidth to ensure proper reception and decoding of the information.
In yet another method of reducing peak to average power ratio, the output signal of an IFFT of all the signal components is scaled to bring the peaks below the maximum level. A problem with this solution is that the signal to noise ratio is reduced proportionally with the scaled factor. Reducing the signal to noise creates a great number of other problems which makes this method unattractive. For example, as the signal to noise ratio decreases more errors occur during transmission.
What is desired is a method of reducing the peak to average power ratio of a transmission within a multi-carrier communication system. A method without a significant decrease in the amount of usable bandwidth, and with low complexity such that reduction of the peak to average power ratio may be performed in real time, is also desirable.